1. Field of the Invention
The present invention relates generally to a variable damping characteristics shock absorber for an automotive suspension system, which has variable operational modes for adapting vehicular suspension characteristics to a vehicle driving condition. More specifically, the invention relates to a shock absorber which has a compact sensor and an actuator mechanism for monitoring the vehicular body vibration mode and the switching operational mode in order to achieve both riding comfort and driving stability of the vehicle.
2. Description of the Background Art
In the recent automotive technologies, respective automotive components have required high performance and high responsiveness. In the case of a suspension system, it has been a high level of riding comfort and driving stability has been required. In order to achieve both the high level of riding comfort and the high level of driving stability, a high response against vibration input to the suspension system has been required.
One of the typical variable damping force suspension systems is disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-85210. In the disclosed system, a piezoelectric element is disposed in each shock absorber in each suspension system for detecting the variation of fluid pressure in the shock absorber. A control unit is provided for receiving an input indicative of the fluid pressure provided from the piezoelectric element. The control unit outputs a controlled voltage to the piezoelectric element for switching the operation mode of the shock absorber at least between a SOFT mode, in which a smaller damping force is to be generated in response to vibration input, and a HARD mode, in which greater damping force is to be generated in response to vibration input.
In general, the control unit is responsive to low frequency input vibration which induces an attitude change of the vehicle body to switch the operational mode of the shock absorber into the HARD mode for a given period of time. While the shock absorber is maintained at the HARD mode, the piezoelectric element maintains operation as an actuator for maintaining a HARD mode operation of the shock absorber. Such manner of control of the suspension characteristics may be effective in terms of suppression of vehicular attitude change, such as rolling and/or pitching. However, on the other hand, even in response to road shock input from a vehicular wheel, it is also desirable to adjust the suspension characteristics depending upon the nature of the road shock in order to satisfactorily achieve both riding comfort and driving stability. Therefore, the prior proposed variable damping characteristics suspension systems for automotive vehicles are not at all satisfactory.
Furthermore, in the prior proposed system, while it is active as the actuator, the piezoelectric element cannot monitor fluid pressure.
In the modern technology of suspension control, it has been considered that the varying of damping characteristics of shock absorber between a piston compression stroke in response to a bounding motion between the vehicle body and a road wheel and a piston expansion stroke in response to a rebounding motion between the vehicle body and the road wheel in order to obtain better vibration stabilizing performance is required. Therefore, it is desirable to adjust the damping characteristics of the shock absorber depending upon the mode of piston action. In order to realize this adjustment, it is essential to detect the piston action mode on the basis of the variation of the fluid pressure in the shock absorber. However, as set forth above, the piezoelectric element is held inoperative as the fluid pressure sensing element, while the shock absorber is maintained in the HARD mode.
This may cause a problem in damping shocks. For example, when the damping characteristics in the HARD mode are set to generate a relatively great damping force in response to vibration input, the damping force generated in response to the piston compression mode action can amplify the input vibration. This tendency may be significant for the second and subsequent vibration cycles. This clearly degrades the vibration stabilizing performance of the vehicle to provide a feeling of a rough ride.
In addition, in the prior proposed suspension system operates substantially a passive manner to detect the input vibration based the variation of the fluid pressure in the shock absorber. Namely, until the vehicle driving condition is changed to require switching of the suspension characteristics, the suspension system will never operate to switch suspension characteristics. Therefore, when a criterion of the fluid pressure to switch the suspension mode from the SOFT mode to the HARD mode is set at a relatively high value in order to provide better riding comfort, the switch from the SOFT mode to the HARD mode tends to be delayed causing bottoming due to low response characteristics, particularly at the initial stage of the switching of suspension mode.
In view of the shortcoming or drawback in the prior art, the co-pending U.S. patent application Ser. Nos. 337,349, 340,062, 359,091, now U.S. Pat. No. 4,948,163, and 388,006 respectively filed on Apr. 13, 1989, Apr. 18, 1989, May 30, 1989 and Aug. 1, 1989, and assigned to the common assignee of the present invention, propose variable damping characteristics shock absorbers which can solve the foregoing problems in the prior art. These prior proposed shock absorbers resolve most of the shortcoming and drawback in the prior art and are thus effective for using in the automotive suspension system.
For prior proposed shock absorbers, it is further desirable to reduce the size and complexity for facilitating more compact units so as to expand the applicability of the shock absorber for greater variation of vehicular suspension structures.